Microbial opsins are naturally occurring seven-transmembrane proteins that, in response to light, translocate ions into or out of cells, and thus when heterologously expressed in genetically targeted excitable cells, enable their membrane potential to be controlled by light [Bernstein, J. G., et al. Curr Opin Neurobiol, (2011) Vol 22, No. 1, 2012, pp 61-71; Chow, B. Y. and E. S. Boyden, Sci Transl Med, 2013. 5(177): p. 177 p5; and. Boyden, E. S., F1000 Biol Rep. (2011) 3:11. Epub 2011 May 3]. As a result, such “optogenetic” tools have found widespread use in fields such as neuroscience for enabling optical activation or silencing of the electrical activity of cells [Bernstein, J. G., et al. Curr Opin Neurobiol, (2011) Vol 22, No. 1, 2012, pp 61-71], and have even been contemplated as building blocks of new kinds of optical biological control therapeutic [Chow, B. Y. and E. S. Boyden, Sci Transl Med, 2013. 5(177): p. 177 p5]. For example, the light-gated cation channel channelrhodopsin-2 (ChR2), which upon expression in neurons enable them to be electrically depolarized using light [Boyden, E. S., et al., Nat Neurosci, 2005. Vol. 8(9): p. 1263-8; Boyden, E. S., F1000 Biology Reports, 2011 3:11].
The light-activated cation channel ChR2 passes four endogenous species of positively charged ion (sodium, potassium, calcium, hydrogen) into cells, possesses defined on kinetics in response to light and off kinetics after cessation of light, passes current with an amplitude governed by its conductance, and responds to light of specific colors.
Although many users of channelrhodopsins are focusing on short-term optical activation to drive spiking activity, the calcium ions and protons that permeate the channelrhodopsin can in principle drive signaling processes within cells, ranging from kinase and phosphatase activation, to cell survival and death, to receptor trafficking, to synaptic plasticity, to gene transcription (e.g., [Neher, E. and T. Sakaba, Neuron, 2008. 59(6): p. 861-72; Ghosh, A. and M. E. Greenberg, Science, 1995. 268(5208): p. 239-47; Orrenius, S., et. al. Nat Rev Mol Cell Biol, 2003. 4(7): p. 552-65.; Chesler, M., Physiol Rev, 2003. 83(4): p. 1183-22 1.; Graef, I. A., et al., Nature, 1999. 401(6754): p. 703-8.; Hardingham, G. E., et al., Nature, 1997. 3 85(6613): p. 260-5.; Dolmetsch, R. E., et al. Nature, 1998. 392(6679): p. 933-6.; Giese, K. P., et al., Science, 1998. 279(5352): p. 870-3.; Malenka, R. C., et al., Science, 1988. 242(4875): p. 8 1-4.; Hayashi, Y., et al., Science, 2000. 287(546 1): p. 2262-7.; Zucker, R. S., Curr Opin Neurobiol, 1999. 9(3): p. 305-13.; and Xia, Z., et al., J Neurosci, 1996. 16(17): p. 5425-36.]. Thus, for many specific scientific purposes, channelrhodopsins that pass the relatively biochemically inert monovalent sodium and potassium currents might be valued, in order to focus the optical effect on depolarization of the cell.